[Bug tree-optimization/44563] GCC uses a lot of RAM when compiling a large numbers of functions

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44563

--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
Funnily apart from the IPA inline summary updating issue the next important
time-hog is basic-block splitting we do for inlining a call.  This is because
split_block moves the tail of the block to a new one (and we process inlines
from BB head to BB start).  And thus we hit gimple_set_bb () quite hard
(quadratic in the number of calls in main() which is composed of a single BB).

So in theory it's better to work backwards from the BB - but that doesn't play
well with catching all BBs in gimple_expand_calls_inline and its caller.

We are talking about 8% of compile-time spent in gimple_set_bb here (according
to callgrind).

It's bad that splitting blocks is O(n) (but that stmt -> bb pointer saves us
in other places).

If we'd know that we perform multiple inlinings in a block we could use a
special "split block" function that splits the block in multiple places
at once, avoiding the quadraticness seen here.  Basically first split
all calls that we want to inline to separate blocks and then do the actual
inlining run.